1. Field of the Invention
The present invention relates to an optical sheet and a display device having the optical sheet. More particularly, the present invention relates to an optical sheet capable of diffusing and condensing light, and a display device having the optical sheet.
2. Description of the Related Art
Generally, a liquid crystal display (LCD) is a non-emissive type display device, so that the LCD device necessarily requires a light source such as a backlight device to supply a backside of an LCD panel of the LCD device with light.
The backlight assembly may be classified as either an edge illumination type backlight assembly or a direct illumination type backlight assembly. In the case of the edge illumination type backlight assembly, a light source such as cold cathode fluorescent lamps (CCFL) are disposed adjacent to a side edge surface of a light-guide plate (LGP). In the case of the direct illumination type backlight assembly, a plurality of light sources is disposed at a rear surface of the LCD panel.
The backlight assembly includes a plurality of optical sheets that enhance optical characteristics of light exiting from the light source. The optical sheet may include a diffusion sheet which diffuses light, a prism sheet which condenses the diffused light toward a front direction, a protective sheet which protects the prism sheet, etc.
In order to improve luminance uniformity and general luminance levels, additional diffusion sheets, prism sheets, and/or reflective polarizing sheets, etc., may be used in the display device. However, the additional sheets may cause a cost increase for manufacturing the backlight assembly. It is desirable to improve luminance without additional costs due to the extra sheets. In order to decrease the number of optical sheets, when the diffusion sheet and the prism sheet are replaced with one optical sheet, the one optical sheet should have both functions of the diffusion sheet and the prism sheet.
Moreover, the one optical sheet should not decrease display quality, as may be the case with multiple optical sheets. Particularly, the one optical sheet should satisfy certain visibility conditions, such as luminance and contrast ratio characteristics of the display screen, TCO'03 conditions, etc. For example, a TCO'03 condition may represent a required condition in which a luminance ratio should be no more than 1.7 at two edges of the display panel when the luminance of the edges of the display panel is observed from a predetermined viewing angle. (TCO stems from the Swedish language and stands for “Tjänstemännens Centralorganisation,” and the English translation is the “Swedish Confederation of Professional Employees.”)
As described above, there are various technologies that include using one optical sheet having multiple functions. For example, the multifunction optical sheet includes a transparent film, a plurality of prism patterns or a plurality of lenses formed on an upper surface of the transparent film and a plurality of diffusion beads formed under a lower surface of the transparent film. For another example, the multifunction optical sheet includes a plurality of transparent layers having different refractive indices. For still another example, the multifunction optical sheet includes a light-diffusion layer having an air bubble formed therein.
However, the multifunction optical sheet developed in accordance with conventional technology may not fully satisfy required display conditions that are achieved by a plurality of optical sheets. Particularly, the conventional multifunction optical sheet may not include a structure for satisfying visibility conditions of a display panel, such as the TCP'03 conditions.
Thus, an optical sheet having multiple functions capable of replacing optical sheets used for a display device is required to be developed, which satisfies display quality conditions, such as luminance, contrast ratio and TCO'03 conditions that are required by the display device.